Many digital video and still image capture systems use a image sensor that is constructed from a complementary metal oxide semiconductor (CMOS) process. CMOS image sensors offer the ability to integrate digital image signal generation circuits directly onto the sensor to achieve a lower system cost. The image sensing portion of a CMOS sensor is constructed of an array of light sensitive elements, each commonly referred to as a “pixel” element. Each pixel element is responsible for capturing one of three color channels: red, green, or blue. Specifically, each pixel element is made sensitive to a certain color channel through the use of a color filter placed over the pixel element such that the light energy reaching the pixel element is due only to the light energy from a particular spectrum. Each pixel element generates a signal that corresponds to the amount of light energy to which it is exposed.
Advances in CMOS processing technology allows the shrinking of the surface area of each pixel element to create higher densities of these light sensors in sensor arrays, thus arriving at sensors with greater resolutions. However, as the numbers of metal layers increases to accommodate the additional circuitry necessary to support the greater number of pixels, thereby so does the vertical heights of pixel elements increase. For example, in a pixel element that has multiple metal layers, the stack height, including the color filter and microlens layers, reaches approximately 10 μm. As light sensitive elements are constructed on the silicon layers underneath the metal layers, light will have to travel on the order of 10 μm from the topmost metal layer to the silicon interface through the opening in the metal layers. As the photosensitive area for a pixel element is reduced, the opening for the pixel element begins to resemble a pinhole. The increased vertical length from the opening to the photosensitive area, coupled with the decrease in opening size, introduces shadowing and interference effects that limit the amount of light incident upon the pixel surface.
The limitation on the range of acceptance angles from which light can incident upon the sensor limits camera performance. Microlenses usually help and produce larger acceptance angle. Using an improved taking lens may improve camera performance, as the taking lens can better channel incident light to strike the sensor at a better angle. Although using a better taking lens may improve camera performance, the increased cost of using an improved taking lens limits the amount of compensation that can be achieved by such use. Thus, it would be desirable to improve light exposure on the sensor without an increase in cost.